1. Field
Embodiments relate to a material for an organic photoelectric device, and an organic photoelectric device including the same.
2. Description of the Related Art
A photoelectric device is, in a broad sense, a device for transforming photo energy to electrical energy, and conversely, for transforming electrical energy to photo energy. The photoelectric device may be exemplified by an organic light emitting diode, a solar cell, a transistor, and so on.
Particularly, among these photoelectric devices, the organic light emitting device employing organic light emitting diodes (OLED) has recently drawn attention due to the increase in demand for flat panel displays.
The organic light emitting device transforms electrical energy into light by applying current to an organic light emitting material. It has a structure in which a functional organic material layer is interposed between an anode and a cathode.
The organic light emitting diode has similar electrical characteristics to those of light emitting diodes (LEDs) in which holes are injected from an anode and electrons are injected from a cathode, then the holes and electrons move to opposite electrodes and are recombined to form excitons having high energy. The formed excitons generate light having a certain wavelength while shifting to a ground state.
In 1987, Eastman Kodak, Inc., developed an organic light emitting diode including a low molecular weight aromatic diamine and an aluminum complex as an emission-layer-forming material (Applied Physics Letters. 51, 913, 1987). C. W. Tang et al. disclosed a practicable device as an organic light emitting diode in 1987 (Applied Physics Letters, 51 12, 913-915, 1987).
The organic layer may have a structure in which a thin film (hole transport layer (HTL)) of a diamine derivative and a thin film of tris(8-hydroxy-quinolate)aluminum (Alq3) are laminated. The Alq3 thin film of Alq3 functions an emission layer for transporting electrons.
Generally, the organic light emitting diode is composed of an anode of a transparent electrode, an organic thin layer of a light emitting region, and a metal electrode (cathode) formed on a glass substrate, in that order. The organic thin layer may include an emission layer, a hole injection layer (HIL), a hole transport layer (HTL), an electron transport layer (ETL), or an electron injection layer (EIL). It may further include an electron blocking layer or a hole blocking layer according to the emission characteristics of the emission layer.
When an electric field is applied to the organic light emitting diode, the holes and electrons are injected from the anode and the cathode, respectively. The injected holes and electrons are recombined on the emission layer though the hole transport layer (HTL) and the electron transport layer (ETL) to provide light emitting excitons. The provided light emitting excitons emit light by transiting to the ground state.
The light emitting material may be classified as a fluorescent material including singlet excitons and a phosphorescent material including triplet excitons.
The phosphorescent light emitting material may be useful as a light emitting material (D. F. O'Brien et al., Applied Physics Letters, 74 3, 442-444, 1999; M. A. Baldo et al., Applied Physics letters, 75 1, 4-6, 1999). Such phosphorescent emission occurs by transition of electrons from the ground state to the exited state, non-radiative transition of a singlet exciton to a triplet exciton through intersystem crossing, and transition of the triplet exciton to the ground state to emit light.
When the triplet exciton transitions, it cannot directly transition to the ground state. Therefore, the electron spin is flipped, and then it transitions to the ground state. Thus, it provides a characteristic of extended lifetime (emission duration) relative to that of fluorescent emission.
In other words, the duration of fluorescent emission is extremely short (at several nanoseconds), but the duration of phosphorescent emission is relatively long (such as at several microseconds), so that phosphorescent emission provides a characteristic of extending the lifetime (emission duration) to more than that of the fluorescent emission.
Quantum mechanically, when holes injected from the anode are recombined with electrons injected from the cathode to provide light emitting excitons, the singlet and the triplet are produced in a ratio of 1:3, in which the triplet light emitting excitons are produced at three times the amount of the singlet light emitting excitons in the organic light emitting diode.
Accordingly, the percentage of the singlet exited state is 25% (the triplet is 75%) in the case of a fluorescent material, so it has limits in luminous efficiency. On the other hand, in the case of a phosphorescent material, it can utilize 75% of the triplet exited state and 25% of the singlet exited state, so theoretically the internal quantum efficiency can reach up to 100%. When phosphorescent light emitting material is used, it has advantages in an increase in luminous efficiency of around four times that of the fluorescent light emitting material.
In the above-mentioned organic light emitting diode, a light emitting colorant (dopant) may be added in an emission layer (host) in order to increase the efficiency and stability in the emission state. In this structure, the efficiency and properties of the light emission diodes are dependent on the host material in the emission layer.